1. Field of the Invention
Measuring the activity in the information storage units of a computer.
2. Description of the Prior Art
The performance of a computer is important because of its high fixed cost, its high operational cost, and its potential for processing information at high speed. Poor performance, in terms of utilization, will not only result in higher operational costs through less work per unit time but also in higher fixed costs through additional equipment needed to handle the work.
Since the theoretical maximum processing capacity of the computer is difficult to determine because of the number of factors affecting it, an assumed maximum capacity is used as an indicator to determine performance. However, determining performance is not easy.
One method is to determine the amount of time, and therefore the percentage of time, that various electrical circuits within the computer are operating. This is done by two principal methods.
The first is to connect wires to specific probe points within these circuits. The wires are connected to external recording devices which will then show when current, and therefore information, is passing through the circuit.
For example, an electrical circuit that would be monitored is the circuit between information storage and either the computer processing section or an external source. The greater percentage of time that information is passing through this circuit, as indicated by the current within the circuit, the better the performance of the computer.
In most large computers the information, both processable information and processing or program information, is stored on discs in an array of cylinders. The cylinders are best represented by concentric rings on the surfaces of the discs. There may be as many as five hundred cylinders of information on a disc and five thousand or more bits of information on each cylinder. Usually, a number of discs, from two to twenty, are mounted together as a unit and rotate on a single axis. There may be as many as one thousand of these units in a computing facility. Three to four hundred are common.
For each side of a disc there is a corresponding read-write head. The head reads or carries information stored on a cylinder to the computer information processing section or to an external source, and writes or carries information either from an external source or the computer information processing section to a cylinder for storage.
Each head is mounted on an arm. The arms for a unit of discs are integral and form a comb. The comb moves all tbe heads radially back and forth together among the cylinders on the discs. Only information to or from a single cylinder on a unit of discs may be transferred or accessed at any one time.
One method of determining the performance of the disc storage, and the computer, is to attach wires to the electrical circuits of the heads and determine the amount of time that current is passing through the heads. There are a number of problems that can occur when wires are attached to these circuits, or to any electrical circuit within the computer. The major problem is that the information passing through the circuit may be changed when external wires are attached. Extraneous current from the wire may pass into the circuit and be interpreted as information by the computer information processing section or the storage system. If there is no information passing through the circuit, the extraneous current may supply information when there shou;d be none. If there is current, and information, passing through the circuit, the extraneous current may add or subtract from that current and change the information passing through the circuit. These changes will result in reduced performance because the information must be reprocessed. Often the computer will stop because the information or program is unprocessable.
It is also difficult to place the wires properly. Several man days are required because of the number of wires. The probe points must be found and tests made to determine whether they are located correctly.
The possibilities for error are great when testing for performance this way.
A second method is to make a continuing record of the transactions occurring in the system. A software program is introduced into the system. This program requires that any transactions be recorded on a separate tape or disc file. The location of the transaction will also be recorded. The file is later analyzed to determine the performance of the computer. A practical problem is that the analysis occurs long after the transactions that were monitored, and the conditions within the computer and information storage at the time of analysis may not be the same as at the time of monitoring. It is an attempt to improve the performance of a filing system after many of the old files, on which the performance evaluation has been based, have been replaced with new files.